Lightning protection apparatus and method

ABSTRACT

A deployable lightning protection apparatus particularly suited for use with an aircraft radome comprises a source of electrically conducting fluid, a deliver apparatus which delivers the conducting fluid flow across to the surface of the radome prior to a lightning strike; and a control unit for controlling the delivery apparatus. The conducting fluid across the outer surface of the radome provides a conductive path for the passage of electrical current resulting from a lightning strike and dissipating said current without damage to the radome. The apparatus allows lightning protection to be deployed in response to a change in atmospheric conditions indicative of a high probability of lightning strike and removed when the danger of lightning strike has passed.

This invention relates to lightning protection apparatus and inparticular but not limited to lightning protection apparatus forradomes, and similar devices.

BACKGROUND OF THE INVENTION

It is well known that electric fields distort around sharp pointscausing a concentration of field strength about such points. As aresult, sharp extremities on tall buildings and air borne vehicles areparticularly prone to being struck by lightning in a storm. This is aparticular problem for aircraft which, in order to be more aerodynamic,often incorporate sharply radiused projections. One example of such asharply radiused projection is the radome which is generally fitted tothe nose of an aircraft.

The radome generally carries the radar system and otherelectro-magnetically sensitive equipment and is by necessity made from adielectric material, consequently a lightning strike to the radome canresult in disintegration of the radome and subsequent in loss of theaircraft through aerodynamic instability. Thus, aircraft are providedwith lightning protection systems to limit the damage which may becaused in the event of a lightning strike to the radome.

Conventional protection systems are known as lightning diverters. Thesegenerally consist of metal strips extending from the tip of the radome,across its external surface and back towards the metal airframe of theaircraft. When lightning strikes the radome, the current is carried bythe conducting strips to the metal airframe where higher currentdensities can be safely dissipated. More recent variations comprise whatis known as a button strip. This consists of a row of closely spacedmetal dots carried on a strip of dielectric material. Just prior to alightning strike, the atmospheric electric charge surrounding theaircraft builds, the dielectric begins to ionise, thus initiating theelectrostatic ionisation of surrounding air molecules. The metal dotsincrease their local field strength and form a plasma thus providing aconductive channel for conducting the current induced by the lightning.

During lightning strike the small quantities of metals used in theseconductors are subject to extreme temperatures and electro-dynamicforces which tend to cause them to ablate. As a consequence, thesesystems have a “one strike” capability and must be replaced on landing.

SUMMARY OF THE INVENTION

A further problem with these conventional technologies is that theyrequire the presence of metal on the radome at all times, irrespectiveof atmospheric conditions. The conductive properties of this metal cancause serious aberration of radar system radiation patterns, withconsequent degradation in the system's performance.

The present invention provides a lightning protection apparatus for aradome comprising;

a source of electrically conducting fluid;

a delivery means for delivering the conducting fluid to the surface ofthe radome prior to a lightning strike;

a control means for controlling the delivery system; and

means for directing the conducting fluid across the outer surface of theradome thereby providing a conductive channel for the passage ofelectrical current resulting from a lightning strike and dissipatingsaid current without damage to the radome.

The provision of the electrically conductive medium in a fluid formpermits a flexible system whereby the lightning conductive element canbe deployed as and when atmospheric conditions are such that there is asignificant risk that lightning may strike. The control means monitorsthe atmospheric condition and initiates delivery of the conductive fluidthrough the delivery means to the surface of the radome when a changeindicative of a high probability lightning strike is detected.

Airflow over the radome surface during flight is sufficient to carry theconductive fluid across the radome surface and direct it towards theairframe thus providing a channel for conducting any current induced bya lightning strike to the airframe for dissipation. When conditions aresuch that there is no significant danger of lightning strike, theconductive fluid can be removed from the radome surface. When lightningprotection is not needed, the conductive fluid can be stored in aninsulating container thereby removing the conductive interference fromthe radar system and any consequent degradation of radar performance.

The control means will generally comprise a series of sensors fordetecting changes in the atmosphere associated with imminent lightning.These sensors may detect factors such as changes in light levels,temperature, humidity and the like but most preferably detect changes inelectrostatic field strength. Preferably, threshold sensors are alsoincorporated into the control means for recognising when the fieldstrength has exceeded a predetermined level indicative of a highprobability of lightning strike. The control means may additionallyincorporate software for controlling the delivery and removal of thefluid. Typically, a predetermined threshold level would be in the regionof 1000 volts per meter.

In some circumstances, aircraft are known to accumulate electrostaticcharge in the course of flight in relatively stable weather conditions.In these circumstances the polarity of the E-field over the entiresurface of the aircraft will be the same (i.e. either directed outwardfrom the surface, or inward towards the surface at all points). In ahigh probability of lightning strike atmosphere, the polarity of theE-field at the aircraft surface will vary over the surface, beingoutward in some regions, and inward in others. Thus, in order to betterdiscriminate high probability lightning strike conditions from strongE-fields due to other phenomena, it is preferred that the control meansincorporate a means for detecting localised polarity of E-fields at theaircraft surface.

In some embodiments of the invention, this is achieved by providing aplurality of polarity sensitive electrostatic field sensors located atdiverse positions on the aircraft surface, preferably in acircumferential spatial arrangement about the longitudinal axis of theradome. It is advantageous to sample the field at surfaces facingupward, downward, left, right, forward and backward. Suitableelectrostatic and polarity sensitive sensors include integrated opticsE-field sensors such as those which utilise Pockel's or Kerr'selectro-optical effects in materials such as Lithium Niobate. A logiccircuit is also incorporated which is configured to recognise acondition where at least one electrostatic sensor detects a fieldamplitude which exceeds the predetermined threshold level and thepolarity of the field detected by each of the plurality of electrostaticfield sensors is not the same. When this condition is recognised, thelogic circuit activates the delivery system by any suitable switchingmechanism.

Whilst airflow is sufficient to direct the conductive fluid across theradome, it may be preferable to provide some form of guide in thesurface of the radome to enable the conductive fluid to travelconsistently in the same path. Such a guide may conveniently be providedin the form of a shallow groove on the surface of the radome.

Once the threat of a lightning strike has passed, it is desirable toremove the conductive channel from the radome surface. Again, once thedelivery means has ceased delivery of the fluid, air flow can be used toremove the fluid from the surface. Preferably, the apparatus willcomprise features specifically designed to remove the fluid. In oneoption, such a feature may comprise a source of clean carrier liquid andmeans for flushing the clean carrier liquid through the delivery systemand over the conductive channel thereby removing the conductive channel.The control means can be configured to recognise changes in thesurrounding atmosphere indicative of a reversion from a high probabilityof lightning strike condition back to a normal condition.

The delivery means itself may comprise any suitable form butconveniently comprises two or more dialectric capillary tubes which ventclose to the tip of the radome and a pump and valve arrangementassociated with a reservoir of the conducting fluid for pumping fluidinto the capillary tubes. The delivery system is conveniently operatedby a pneumatic or hydraulic system and should be electrically andspatially isolated from the conducting airframes or anythingelectrically connected to it, in order to prevent lightning striking theaircraft via a path inside the radome. This may conveniently be achievedby operation via a pneumatic or hydraulic system, employingnon-electrically conducting pipes and fluids. Alternatively, thedelivery means may be operated by electric pump and valve means poweredby a local battery and the control means comprises a signalling circuitof optical fibres.

Where a pump is used to deliver the conductive fluid, the pump may havea reversible action so that the fluid can be withdrawn back into thereservoir when the threat of lightning is removed.

Suitable fluids for use as the conductive fluid include any dielectriccarrier loaded with conducting particles. For example distilled watercarrying carbon particles. Additives which may optionally be added toimprove performance include, wetting agents, anti-blockage agents whichseparate particles to prevent blockage of delivery tubes and orifices,additives for reducing the evaporation temperature or rate ofevaporation of the fluid and anti-static or anti-cling agents tominimise adherence of conductive particles after delivery. Alternativefluids include conductive gases or particulates of conductive materialsuch as mercury vapour or carbon smoke.

In another aspect, the present invention provides a method forconducting lightning across the surface of a non-conducting articlecomprising;

providing a source of electrically conducting fluid;

delivering the conducting fluid to the surface of the article prior to alightning strike; and

directing the conducting fluid across the outer surface of the articlethereby providing a conductive channel for the passage of electricalcurrent resulting from a lightning strike and dissipating said currentthrough a conductive medium.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic flow chart of one embodiment of the inventivesystem;

FIG. 2 illustrates a pneumatically operated embodiment of the invention;

FIG. 3 illustrates an electrically operated embodiment of the invention;

FIG. 4 illustrates the control system for the embodiment of FIG. 2; and

FIG. 5 illustrates the control system for the embodiment of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

As can be seen from FIG. 1, a system of electrostatic sensors indicatedgenerally by reference numeral 1 provide input to a control system 2which comprises a threshold sensor and a simple logic circuit. When thelogic circuit detects conditions indicative of a high probability oflightning strike, it communicates this to the pump 4 and valves 3 of thedelivery system 7, 8. Conducting fluid from a reservoir 5 is transportedthrough valve means 3 and pump 4 to a system of capillary tubes 7 whichvent at various points near the tip of the radome. On deployment of theconducting fluid, as the aircraft is in flight, airflow drags thedelivered conducting fluid in a direction opposing the direction oftravel of the aircraft across the radome surface and towards the metalairframe.

In the particular embodiment shown, a second reservoir 6 of cleancarrier fluid is provided. The control system is configured to detect areversing of conditions to below the threshold value. When thiscondition is recognised the control system communicates this to thevalves 3 which switch to allow release of the clean carrier fluid overthe radome surface thereby removing the conductive path. To preventlightning current being conducted internally to the radome, theconductive fluid reservoir 5, valves 3 and pump 4 are encased in adielectric container 8, the capillary tubes 7 are also made from aninsulating material.

FIG. 2 illustrates an embodiment similar to that of FIG. 1 incorporatedinto the nose portion of an aircraft. As can be seen from the Figure, acapillary tube 7 vents at a point A near to the tip of the radartransparent radome 10 of the aircraft. When a high probability oflightning strike is detected, the conductive fluid is pumped from thereservoir 5 via the valves 3 and pump 4 to the capillary tube 7 which isone of a number of similar tubes. Due to the geometry of the aircraftnose cone and airflow during flight, the fluid travels along the bottomsurface of the radome towards the metal airframe 16 creating aconductive channel 9 leading from the tip of the radome A to a point Bon the conductive airframe 16. A supply of pressurised air is providedvia a dielectric pipe 11 to drive the pneumatic pump 4. Additionaldielectric pipe work 12 carry hydraulic or pneumatic fluid controlled bythe logic circuit to activate the delivery system when a highprobability of lightning strike is detected.

The embodiment shown in FIG. 3 operates in essentially the same manneras that described in relation to FIG. 2, however, in this embodiment,the control system is operated by opto-electric rather than hydraulicand pneumatic means. A battery 14, contained in dielectric container 8powers the pump 4 and valves 3. Information from and to the logiccircuit is relayed via signals through optical fibres 15.

FIGS. 4 and 5 show the basic circuitry for the sensor system. FIG. 4relates to the embodiment shown in FIG. 2 and FIG. 5 relates to theembodiment shown in FIG. 3. An E-field sensor 21 detects a charge in thesurrounding E-field and relays a signal to an amplifier 22. The signalis processed through a low pass filter 23 and to a comparator 24 whereit is compared against a voltage reference 25. Preferably the comparatorcircuit incorporates a polarity identifier. Simultaneously other signalsare relayed by other sensors 21 a, 21 b, 21 c, 21 d through similarcircuits. The comparators 24 relay the signal to the threshold detector2 and logic circuit for processing.

Whilst the foregoing embodiments describe the invention for use inrelation to a radome on an aircraft, the skilled person will understandthat the invention is not limited to these embodiments. The basicprinciple behind the invention, that is, the use of a deployable fluidconductor in place of a permanent solid conductor, may be used toreplace conventional lightning conductors in numerous otherapplications.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

1. A lightning protection apparatus for a radome attached to an airframecomprising: a source of electrically conducting fluid; a delivery meansfor delivering the conducting fluid to a surface of a radome on whichthe protection apparatus is installed, prior to the lightning strike; acontrol means for controlling the delivery means, said control meanshaving at least one electrostatic field sensor for detecting a change insurrounding atmospheric conditions indicative of a high probability of alightning strike; and means for initiating delivery of the conductingfluid on detection of such a change in atmospheric conditions,comprising a threshold detector for detecting when an electrostaticfield amplitude detected by the at least one electrostatic field sensorexceeds a predetermined threshold level, and a switch for activating thedelivery means when the predetermined threshold level is exceeded; andmeans for directing the conducting fluid across the radome surface onwhich the protection apparatus is installed, towards the airframe,thereby providing a channel having a current conducting capacitysufficient for conducting any current induced by a lightning strike tothe airframe for dissipation without damage to the radome.
 2. Alightning protection apparatus for a radome as claimed in claim 1,wherein: a plurality of polarity sensitive electrostatic field sensorsare provided in a circumferential spatial arrangement about thelongitudinal axis of the radome; and the means for initiating deliverycomprises a logic circuit configured to recognize a condition where atleast one electrostatic sensor detects a field amplitude which exceedsthe predetermined threshold level and the polarity of the field detectedby each of the plurality of electrostatic field sensors is not the same,the switch being activated by the logic circuit only when both theseconditions are met.
 3. A lightning protection apparatus for a radome asclaimed in claim 1, wherein the predetermined threshold level isapproximately 1000 volts per meter.
 4. A lightning protection apparatusfor a radome as claimed in claim 1, wherein the means for directing theconducting fluid across the outer surface of the radome comprisesgrooves on the surface of the radome.
 5. A lightning protectionapparatus for a radome as claimed in claim 1, further comprising meansfor deactivating the conductive channel when the surround atmosphericconditions are no longer indicative of a high probability lightningstrike.
 6. A lightning protection apparatus for a radome as claimed inclaim 5, wherein the means for deactivating the conductive channelcomprises: a source of clean carrier liquid; and means for flushing theclean carrier liquid through the delivery system and over the conductivechannel, thereby removing the conductive channel.
 7. A lightningprotection apparatus for a radome as claimed in claim 1, wherein thedelivery system comprises: at least two dielectric capillary tubes whichvent close to the tip of the radome; and a pump associated with areservoir of the conducting fluid.
 8. A lightning protection apparatusfor a radome as claimed in claim 1, wherein the delivery systemcomprises a pneumatic or hydraulic system in which all control lines aredielectric and the pneumatic or hydraulic fluid used is not electricallyconducting.
 9. A lightning protection apparatus for a radome as claimedin claim 1, wherein: the delivery means comprises an electric pump and avalve powered by a battery; and the control means comprises a signallingcircuit of optical fibres.
 10. A lightning protection apparatus for aradome as claimed in claim 7, wherein the pump has a forward action fordelivering the conductive fluid to the surface of the radome and areverse action for withdrawing it from the surface of the radome.
 11. Amethod for conducting lightning across a surface of a radome comprising:providing a source of electrically conducting fluid; delivering theconducting fluid to the surface of the radome in response to detectionof a change in surrounding atmospheric conditions indicative of a highprobability lightning strike; and directing the conducting fluid acrossan outer surface of the radome, thereby providing a conductive channelhaving a current conducting capacity sufficient for passage ofelectrical current resulting from a lightning strike and for dissipatingsaid current through an object to which the radome is attached.
 12. Amethod for conducting lightning across the surface of a non-conductingarticle comprising: providing a source of electrically conducting fluid;delivering the conducting fluid to an outer surface of the article priorto a lightning strike; and directing the conducting fluid across theouter surface of the article, thereby providing a conductive channelhaving a current conducting capacity sufficient for passage ofelectrical current resulting from a lightning strike and for dissipatingsaid current through a conductive medium to which the article iselectrically coupled.
 13. Radome apparatus for an aircraft, comprising:a radome made of a nonconducting material, for mounting on an aircraftbody; and first means for providing a conducting path having a currentcarrying capacity sufficient for conducting electricity from a lightningstrike that impinges on said radome, to said aircraft body; said firstmeans comprising, a source of electrically conducting fluid; a deliverymeans for delivering the conducting fluid to a surface of a radome onwhich the protection apparatus is installed, prior to the lightningstrike; a control means for controlling the delivery means, said controlmeans having at least one electrostatic field sensor for detecting achange in surrounding atmospheric conditions indicative of a highprobability of a lightning strike; and means for initiating delivery ofthe conducting fluid on detection of such a change in atmosphericconditions, comprising a threshold detector for detecting when anelectrostatic field amplitude detected by the at least one electrostaticfield sensor exceeds a predetermined threshold level, and a switch foractivating the delivery means when the predetermined threshold level isexceeded; and means for directing the conducting fluid across a surfaceof the radome toward said aircraft body, thereby providing a flow ofsaid conducting fluid which forms said conducting path.